1. Technical Field
The present invention relates to an image forming apparatus forming an image using toner, such as a copier, a printer, a facsimile machine, a plotter, or a multi-function apparatus having at least one capability of the above devices. The present invention also relates to an image forming method employed in such an image forming apparatus and capable of detecting density of the image formed thereby.
2. Related Art
An image forming apparatus that employs a so-called electrophotographic method to form an image with toner has been known, as disclosed by, for example, JP-H09-62042-A, JP-3825184-B, and JP-2000-98675-A. Such an image forming apparatus forms an image in such a manner that a photoreceptor or an image carrier is uniformly charged by a charger, a latent image is formed on the photoreceptor by an exposure unit based on input image data corresponding to a to-be-formed image, and toner is adhered on the latent image by a developing device to thus render it visible.
Electrophotographic image forming apparatuses are widely used in the print industry and demand for faster, higher-quality apparatuses is rapidly increasing. Of those various quality requirements, uniform density over any given printed page is highly demanded and uniformity in the printed page is a decision factor when a user selects an image forming apparatus. Accordingly, minimizing density fluctuation in the printed page is most important.
Density fluctuation occurs due to various factors, such as an unstable charge due to uneven charging; fluctuation of exposure by an exposure unit; rotation fluctuation and variations in sensitivity of an image carrier such as a photoreceptor; variations in the resistance of a developer carrier such as a developing roller; fluctuation in the charge of the toner; and variations in the transferring of a transfer roller. Among these, it is particularly important to minimize density fluctuation caused by the rotary oscillation of the image carrier or unevenness in the sensitivity of the image carrier because such fluctuation occurs repeatedly within a single page because of its short cycle and is thus readily visible.
A description will now be given of the density fluctuation caused by the rotary oscillation of the image carrier.
In the image forming apparatus employing the electrophotographic method, toner is adhered on the image carrier using an electric field generated by an electric potential difference between the developer carrier and the image carrier. Therefore, when a development gap, which is a distance between the developer carrier and the image carrier, fluctuates due to the rotary oscillation of the image carrier, the electric field also fluctuates and causes image density changes or fluctuations.
The density fluctuation caused by the uneven sensitivity of the image carrier is as follows.
Specifically, when the sensitivity of the image carrier responsive to the exposure fluctuates due to factors such as an environmental change or aging deterioration, even though the exposure is performed at a constant exposure amount, the exposed bright area potential after the exposure of the image carrier fluctuates and the resultant electric field changes, so that the density fluctuation occurs. With regard to the uneven sensitivity of the image carrier, if the image carrier is manufactured using a high-precision production method in order to decrease the number of sensitivity errors, manufacturing costs soar.
As a correction technique for the density fluctuation, an approach is conceivable in which a pattern for the detection of density fluctuation is generated and correction data is obtained, so that process conditions such as charging bias, developing bias, and exposure may be changed based on the profiles of the density fluctuation due to the rotation cycle or the uneven sensitivity of the image carrier.
Accordingly, an approach is conceivable in which a developing bias is modulated responsive to a rotation cycle of the image carrier. Specifically, a rotational position detection sensor to detect a rotational position of the image carrier and a density sensor to detect a density of the image are used; a density fluctuation detected by the density detection sensor is divided by the cycle of the image carrier; and the developing bias is cyclically changed with a signal of the rotational position detection sensor as a trigger so that the electric field fluctuation due to the rotary oscillation is cancelled and the electric field becomes constant in order to minimize the detected density fluctuation.
To achieve the above approach, for example, in addition to the developing bias, the charging bias may also be modulated.
Density fluctuation caused by the uneven sensitivity of the image carrier also has other causes.
Specifically, the sensitivity of the toner adhering amount responsive to the electric field changes depending on the image density. That is to say, the sensitivity of the image carrier also changes depending on the image density. Specifically, in the shadow portion that is a high density portion such as a solid image with a high toner adhering amount, the difference in the potential between the exposed bright area potential and the developing bias, that is, the developing potential, becomes a dominant factor. Conversely, in a halftone or highlight image with less toner adhering amount than that of the shadow portion, the difference in the potential between the dark area potential which is the potential of a non-exposed portion of the image carrier and the developing bias, i.e., a background potential, is a dominant factor.
Accordingly, if the developing bias is controlled so as to correct the density fluctuation in the shadow portion, the control effect cannot be obtained in the halftone or highlight portion image and the density fluctuation increases.
JP-H09-62042-A discloses a technique to comprehensively decrease stripe-shaped density fluctuations that are cyclically generated in an image. This technique relates to an image forming apparatus employing an electrophotographic method or electrostatic recording process including a first fluctuation data storage means to previously store the cyclical density fluctuations data of the image density; and a first control means to control the image forming condition based on the density fluctuations data, in which the first fluctuation data storage means stores at least the density fluctuations data corresponding to one cycle of the developer carrier, and the first control means controls at least one of the charged voltage, the exposure light amount, the developer voltage, and the transfer voltage, whereby the density is corrected by the control means in accordance with the rotation cycle of the image carrier.
Alternatively, JP-3825184-B and JP-2000-98675-A disclose a technique to minimize the density fluctuation focusing on the rotation cycle of the developer carrier, not on the image carrier, thereby enabling a reduction of the image density fluctuation occurring at a developing roller rotation cycle by changing the developing bias responsive to the developing roller rotation cycle. Specifically, the developing bias is controlled by performing detection of the density fluctuation based on the image pattern formed on the image carrier and by adjusting each phase of the detected density fluctuation data and the developing roller rotation.
However, the above technique has a disadvantage in that, if the developing bias alone is controlled, even though the solid density correction is performed satisfactorily, the halftone density correction cannot be performed well.
To correct the density fluctuation due to the rotary oscillation of the image carrier, when the image pattern is formed so that the process condition is changed based on the rotation cycle of the image carrier, the length of the image pattern in the sub-scanning direction needs to be lengthened in general. As a result, such disadvantages will occur that the toner consumption amount increases, the load on the cleaning device increases, down time is lengthened, and the like.
When the rotational position detection to detect the rotational position of the image carrier and the density detection sensor to detect the image density are used as described above, detection of the formed image pattern by the density detection sensor is performed such that an average of the image patterns of n-cycles of the image carrier is obtained with reference to the rotational position detection sensor, and correction data is generated and stored.
In this case, there is a case in which the detection signal of the rotational position detection sensor does not come at a head of the image pattern due to the rotation start position of the image carrier to form the image pattern, so that the data acquisition becomes inadequate. To avoid such a situation, if the length of the image pattern in the sub-scanning direction is set to (n+1) cycles of the image carrier so as to securely detect the image pattern at a detection timing of the rotational position detection sensor, useless data is generated in the average acquisition. For example, substantially one cycle data of the image carrier becomes useless and toner used for the image pattern not used for the average acquisition becomes a waste, leading to a toner yield problem.